The respiration of naive F344 rats confined in nose-only inhalation exposure tubes was measured to obtain data for normal adult rats of different ages and to evaluate the tubes for exposures lasting several hours. Exposure tubes were modified for use as volume-displacement plethysmographs. Respiration of 10 male and 10 female rats at 3, 6, 12 and 24 months of age was measured in the tubes during simulated exposures of up to 6 h duration. Measurements included respiratory frequency, tidal volume, minute volume and body surface temperature. The mean respiratory frequencies of 3, 6, 12 and 24 month old rats during the first hour of exposure were 172, 152, 123 and 136 breaths min-1, respectively. Minute volumes were 1.40, 0.89, 0.67 and 0.82 ml g-1 body weight, respectively. Both frequency and minute volumes g-1 body weight were significantly greater for the youngest group, declined with age to 12 months and then increased at 24 months. Minute volumes g-1 body weight were similar for males and females. Minute volume and respiratory frequency of 3 and 12 month old rats declined progressively between 1 and 6 h of confinement in the tubes. Surface temperature did not increase after the first hour. The age and sex-specific data provide a basis for predicting respiration of naive tube-confined rats during inhalation exposures to non-irritating materials.